I've always said that one of the great and terrifying things about the internet is that it allows all of the niche people to find each other. This means chat rooms and message boards that 99% of the population can't relate to, and online stores where you can buy pretty much anything.
Cannibalism, for one reason or another, has never been outlawed in 49 of the 50 states (Idaho being the exception). It's also something that crops up quite a bit in pop culture, usually when there needs to be some way for the antagonist to stand out - see Silence of the Lambs or The Hills Have Eyes. Alternately, there are stories - both fiction and non-fiction - about people who have had to resort to cannibalism to stay alive. Part of the reason I see a business opportunity in cannibalism is that it's one of the few taboos that we have left. If our society has proven anything, it's that we love to break our taboos.
So if you want to sell human flesh for consumption, I see two basic ways to go about it: either you open up a restaurant, or you sell the meat online. But before I go over the benefits and drawbacks to that, let's talk law.
While cannibalism itself might not be illegal, there are a huge host of laws concerning what's to be done with human remains, not to mention food safety laws. What this basically means is that you will need someone legally allowed to handle human remains (a list which includes morticians, policemen, medical examiners, forensic specialists, and other people in the medical field). The other problem is that it's illegal to sell or buy human remains. So a business that is established with just that purpose runs into a little bit of trouble. One of the standard tricks of prostitution is to redefine the service being performed into something else that's of no legal consequence. A masseuse who gives happy endings is being paid for her time, not for the sexual act. This isn't a very convincing argument, but it has kept prostitutes and other sex workers from jail time if the judge is lenient enough. Translating that to the sale of flesh, you would have to advertise it as complimentary to something else - like, say, a free gift that comes with a t-shirt.
So let's say that you want to start a restaurant. Your biggest hurdle is probably finding a location, and once you have one, keeping that location. I imagine that especially at the beginning, public pressure would be on you to move out once people realized what was going on. There would be news stories, protests, etc. In addition to that, you would need more staff - a chef, waiters, that kind of thing - and all of them would have to be okay with the idea of cannibalism and the reality of working with human remains every day. A restaurant also has a physical location, which means that you're cutting yourself off from a large amount of the population. However, there is some precedent in New York, where a chef made cheese out of his wife's breast milk. The New York Department of Health shut that down fairly quickly (and he was just giving it away, not selling it), but you can see the strategy that would have to be taken; the restaurant would sell other dishes as its main product, with the long pig being a specialty to draw in other business.
The other option is the internet. The great thing about the internet is that it's reasonably anonymous, which is why pretty much every dark thought that's ever passed through someone's head has its own private place online. This includes all manner of niche things - this is why Rule 34 exists. Because of the anonymity, people would be able to buy the meat without feeling social stigma for breaking the cannibalism taboo. Because the internet has no physical location, the business would be able to extend across the country, assuming that relevant laws about transporting human remains across state lines could be properly observed. And since it wouldn't have to be in a place with a large population, the business could be incorporated in whichever state has the most lenient laws on human remains.
So here comes the next inevitable question, which you might have been wondering since the beginning of this post: where is this flesh coming from? There are a few options that don't actually involve having someone die. Tumors get removed all the time, and limbs are occasionally unable to be reattached. The problem here is in finding someone who would be willing to sell those things to the company for someone else to eat. I have no doubt that those people exist, but probably not in enough quantity or regularity to base a business off of them. I feel it wouldn't hurt to pursue people with body identity integrity disorder, but again, there's the issue of quantity and regularity. This option is good, because no one can claim that the business is built on death.
A second option is to use flesh grown in labs. Since I'm not a biologist, I can't really speak to how difficult it would be to actually grow muscle (assuming that's what people want to eat). Tengion is already growing artificial organs for transplant. At any rate, it's something that will become easier with time, given that there are a huge number of medical technologies that result from the basic ability to grow parts of people. There would be less of a question about the safety and health issues of eating the meat. It would also remove some of the stigma of cannibalism, because it was never part of a person. However, this is a question of feasibility, because even if it's remotely possible now, it's sure to be damn expensive. In another thirty years, it might be possible to do from a garage.
And finally, we come to dead people. Dead people are a good source of flesh mostly because there are so many of them. Besides, through organ donor programs there are a huge number of people who are willing to promise away parts of their body for no monetary compensation. It's not too ridiculous to believe that people would promise away their flesh in return for a few hundred dollars, especially young people who are strapped for cash. Bringing contracts into the mix also allows for the use of more elaborate legal constructions which help to ensure that no lawsuits are filed against the business. Seeing as organ donation doesn't normally take the edible parts, there could be quite a bit of overlap between the two practices; both of them use parts of the body that would otherwise go to waste, and having them side by side allows for beneficial comparisons.
I think the bigger question here is whether the demand actually exists to support the costs that a business of this nature would entail, but I suppose that's a question that will only be answered when someone makes the effort.